WriterPrototype use non-exist zero property

[dreamershl]

Is it possible to add the string type support as before? Since javascript doesn't support long natively, my app always use the Long.toString(). When encode the message, the previous version will use the Long to parse the string back to Long. But in the new version, this can't be supported anymore.

1. longbits.js doesn't export this zero

    var zero = new LongBits(0, 0);
   

2. writer.js directly use it. when the value isn't valid uint64, the exception will be thrown

   WriterPrototype.uint64 = function write_uint64(value) {
      var bits;
      if (typeof value === 'number')
         bits = value ? LongBits.fromNumber(value) : LongBits.zero;
      else if (value.low || value.high)
         bits = new LongBits(value.low >>> 0, value.high >>> 0);
    else
       bits = LongBits.zero;
    return this.push(writeVarint64, bits.length(), bits);
     };